Communications system and method with non-blocking shared interface

ABSTRACT

A communication system. One embodiment includes at least two functional blocks, wherein an first functional block communicates with a second functional block by establishing a connection. A connection is a logical state in which data may pass between the first functional block and the second functional block. One embodiment includes a bus coupled to each of the functional blocks and configured to carry a plurality of signals. The plurality of signals includes a thread identifier that indicates a transaction stream that the data transfer is part of and a busy signal identified by the thread identifier. The busy signal is issued by the target functional block when resources will be unavailable to perform a transfer.

FIELD OF THE INVENTION

[0001] The present invention relates to a communication system to couple computing sub-systems.

BACKGROUND OF THE INVENTION

[0002] Electronic computing and communications systems continue to include greater numbers of features and to increase in complexity. At the same time, electronic computing and communications systems decrease in physical size and cost per function. Rapid advances in semiconductor technology such as four-layer deep-sub-micron complimentary metal-oxide semiconductor (CMOS) technology, have enabled true “system-on-a-chip” designs. These complex designs may incorporate, for example, one or more processor cores, a digital signal processing (DSP) core, several communications interfaces, and graphics support in application-specific logic. In some systems, one or several of these extremely complex chips must communicate with each other and with other system components. Significant new challenges arise in the integration, verification and testing of such systems because efficient communication must take place between sub-systems on a single complex chip as well as between chips on a system board. One benefit to having an efficient and flexible method for communication between sub-systems and chips is that system components can be reused in other systems with a minimum of redesign.

[0003] One challenge in the integration, verification and testing of modern electronic systems stems from the fact that modern electronic systems in many application areas have functionality, cost and form-factor requirements that mandate the sharing of resources, such as memory, among multiple functional blocks, where functional blocks can be any entity that interfaces to a communication system. In such systems, the functional blocks typically possess different performance characteristics and requirements, and the communications system and shared resources must simultaneously satisfy the total requirements. Key requirements of typical functional blocks are bandwidth and latency constraints that can vary over several orders of magnitude between functional blocks. In order to simultaneously satisfy constraints that vary so widely, communications systems must provide high degrees of predictability.

[0004] Traditional approaches to the design of communications systems for modern, complex computer systems have various strengths and weaknesses. An essential aspect of such approaches is the communications interface that various sub-systems present to one another. One approach is to define customized point-to-point interfaces between a sub-system and each peer with which it must communicate. This customized approach offers protocol simplicity, guaranteed performance, and isolation from dependencies on unrelated sub-systems. Customized interfaces, however, are by their nature inflexible. The addition of a new sub-system with a different interface requires design rework.

[0005] A second approach is to define a system using standardized interfaces. Many standardized interfaces are based on pre-established computer bus protocols. The use of computer buses allows flexibility in system design, since as many different functional blocks may be connected together as required by the system, as long as the bus has sufficient performance. It is also necessary to allocate access to the bus among various sub-systems. In the case of computer buses, resource allocation is typically referred to as arbitration.

[0006] One disadvantage of computer buses is that each sub-system or component connected to the bus is constrained to use the protocol of the bus. In some cases, this limits the performance of the sub-system. For example, a subsystem may be capable of handling multiple transaction streams simultaneously, but the bus protocol is not capable of fully supporting concurrent operations. In the case of a sub-system handling multiple transaction streams where each transaction stream has ordering constraints, it is necessary for the sub-system to identify each increment of data received or transmitted with a certain part of a certain data stream to distinguish between streams and to preserve order within a stream. This includes identifying a sub-system that is a source of a data transmission. Conventionally, such identification is limited to a non-configurable hardware identifier that is generated by a particular sub-system or component.

[0007] Current bus systems provide limited capability to preserve order in one transaction stream by supporting “split transactions” in which data from one transaction may be interleaved with data from another transaction in the same stream. In such a bus, data is tagged as belonging to one stream of data, so that it can be identified even if it arrives out of order. This requires the receiving subsystem to decode an arriving address to extract the identification information.

[0008] Current bus systems do not support true concurrency of operations for a sub-system that can process multiple streams of transactions over a single interconnect, such as a memory controller that handles access to a single dynamic random access memory (DRAM) for several clients of the DRAM. A DRAM controller may require information related to a source of an access request, a priority of an access request, ordering requirements, etc. Current communication systems do not provide for such information to be transmitted with data without placing an additional burden on the sub-system to adapt to the existing protocol.

[0009] In order for many sub-systems to operate in conventional systems using all of their capabilities, additional knowledge must be designed into the subsystems to provide communication over existing communication systems. This makes sub-systems more expensive and less flexible in the event the sub-system is later required to communicate with new sub-systems or components. Existing communication approaches thus do not meet the requirements of today's large, complex electronics systems. Therefore, it is desirable for a communications system and mechanism to allow sub-systems of a large, complex electronics system to inter-operate efficiently regardless of their varying performance characteristics and requirements.

SUMMARY OF THE INVENTION

[0010] One embodiment of the present invention includes a shared communications bus for providing flexible communication capability between electronic sub-systems. One embodiment includes a protocol that allows for identification of data transmissions at different levels of detail as required by a particular sub-system without additional knowledge being designed into the subsystem.

[0011] One embodiment of the invention includes several functional blocks, including at least one initiator functional block and one target functional block. Some initiator functional blocks may also function as target functional blocks. In one embodiment, the initiator functional block is coupled to an initiator interface module and the target functional block is coupled to a target interface module. The initiator functional block and the target functional block communicate to their respective interface modules and the interface modules communicate with each other. The initiator functional block communicates with the target functional block by establishing a connection, wherein a connection is a logical state in which data may pass between the initiator functional block and the target functional block.

[0012] One embodiment also includes a bus configured to carry multiple signals, wherein the signals include a connection identifier signal that indicates a particular connection that a data transfer between an initiator functional block and a target functional block is part of. The connection identifier includes information about the connection, such as which functional block is the source of a transmission, a priority of a transfer request, and transfer ordering information. One embodiment also includes a thread identifier, which provides a subset of the information provided by the connection identifier. In one embodiment, the thread identifier is an identifier of local scope that identifies transfers between an interface module and a connected functional block, where in some embodiments, an interface module connects a functional block to a shared communications bus.

[0013] The connection identifier is a an identifier of global scope that transfers information between interface modules or between functional blocks through their interface modules. Some functional blocks may require all the information provided by the connection identifier, while other functional blocks may require only the subset of information provided by the thread identifier.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a block diagram of one embodiment of a complex electronics system according to the present invention.

[0015]FIG. 2 is an embodiment of a system module.

[0016]FIG. 3 is an embodiment of a system module.

[0017]FIG. 4 is an embodiment of a communications bus.

[0018]FIG. 5 is a timing diagram showing pipelined write transfers.

[0019]FIG. 6 is a timing diagram showing rejection of a first pipelined write transfer and a successful second write transfer

[0020]FIG. 7 is a timing diagram showing interleaving of pipelined read and write transfers.

[0021]FIG. 8 is a timing diagram showing interleaved connections to a single target.

[0022]FIG. 9 is a timing diagram showing interleaved connections from a single initiator.

[0023]FIG. 10 is a block diagram of one embodiment of part of a computer system.

[0024]FIG. 11 is one embodiment of a communications bus.

[0025]FIG. 12 is a block diagram of one embodiment of part of a computer system.

[0026]FIG. 13 is a timing diagram illustrating one embodiment of a credit-based per-thread flow control mechanism.

[0027]FIG. 14 shows an exemplary system consisting of three initiator functional blocks, three target functional blocks, and two communication subsystems that are tied together using a set of interfaces.

[0028]FIGS. 15b and 15 b show examples of a functional block's quality of service guarantees, and a communication subsystem's quality of service guarantees.

[0029]FIG. 16 shows an example interface that has both channel identification and per-channel flow-control signals.

[0030]FIG. 17 shows a flowchart of the quality of service guarantee composition mechanism.

[0031]FIG. 18 shows an embodiment of a process to perform data flow mapping.

[0032]FIG. 19 show an example of unit alignment of quality of service guarantees.

[0033]FIG. 20 shows a sample system with mapped data flows.

[0034]FIG. 21 illustrates one example of how the composition methodology is applied to the system of FIG. 20 to obtain the end-to-end quality of service guarantees.

DETAILED DESCRIPTION

[0035] The present invention is a communications system and method for allowing multiple functional blocks or sub-systems of a complex electronics system to communicate with each other through a shared communications resource, such as a shared communications bus. In one embodiment, a communications protocol allows multiple functional block on a single semiconductor device to communicate to each other. In another embodiment, the communications protocol may be used to allow multiple functional blocks on different semiconductor devices to communicate to each other through a shared off-chip communications resource, such as a bus.

[0036] In one embodiment, the present invention is a pipelined communications bus with separate command, address, and data wires. Alternative embodiments include a pipelined communications bus with multiplexed address, data, and control signals. The former embodiment offers higher performance and simpler control than the latter embodiment at the expense of extra wires. The former embodiment may be more appropriate for on-chip communications, where wires are relatively less expensive and performance requirements are usually higher. The latter embodiment offers higher per-wire transfer efficiency, because it shares the same wires among address and data transfers. The latter embodiment may be more appropriate for chip-to-chip communications between semiconductor devices, because package pins and board traces increase the per signal cost, while total required communications performance is usually lower.

[0037]FIG. 1 is a block diagram of a complex electronics system 100. Shared communications bus 112 connects sub-systems 102, 104, 106, 108, and 110. Subsystems are typically functional blocks including a interface module for interfacing to a shared bus. Sub-systems may themselves include one or more functional blocks and may or may not include an integrated or physically separate interface module. In one embodiment, the sub-systems connected by communications bus 112 are separate integrated circuit chips. Sub-system 104 is an application specific integrated circuit (ASIC) which, as is known, is an integrated circuit designed to perform a particular function. Sub-system 106 is a dynamic random access memory (DRAM). Sub-system 108 is an erasable, programmable, read only memory (EPROM). Sub-system 110 is a field programmable gate array (FPGA). Sub-system 102 is a fully custom integrated circuit designed specifically to operate in system 100. Other embodiments may contain additional sub-systems of the same types as shown, or other types not shown. Other embodiments may also include fewer subsystems than the sub-systems shown in system 100. Integrated circuit 102 includes sub-systems 102A, 102B, 102C, 102D and 102E. ASIC 104 includes functional blocks 101A, 104B and 104C. FPGA 110 includes functional blocks 110A and 110B. A functional block may be a particular block of logic that performs a particular function. A functional block may also be a memory component on an integrated circuit.

[0038] System 100 is an example of a system that may consist of one or more integrated circuits or chips. A functional block may be a logic block on an integrated circuit such as, for example, functional block 102E, or a functional block may also be an integrated circuit such as fully custom integrated circuit 102 that implements a single logic function.

[0039] Shared communications bus 112 provides a shared communications bus between sub-systems of system 100. Shared communication bus 114 provides a shared communications bus between sub-systems or functional blocks on a single integrated circuit. Some of the functional blocks shown are connected to interface modules through which they send and receive signals to and from shared communications bus 112 or shared communications bus 114. Interconnect 115 is a local point-to-point interconnect for connecting interface modules to functional blocks.

[0040] Interface modules 120-127 are connected to various functional blocks as shown. In this embodiment, interface modules 120, 122, 123 and 124 are physically separated from their connected functional block (A, B, C, E and 102, respectively). Interface modules 121, and 125-128 are essentially part of their respective functional blocks or sub-systems. Some functional blocks, such as 102D, do not require a dedicated interface module. The arrangement of sub-systems, functional blocks and interface modules is flexible and is determined by the system designer.

[0041] In one embodiment there are four fundamental types of functional blocks. The four fundamental types are initiator, target, bridge, and snooping blocks. A typical target is a memory device, a typical initiator is a central processing unit (CPU). Functional blocks all communicate with one another via shared communications bus 112 or shared communications bus 114 and the protocol of one embodiment. Initiator and target functional blocks may communicate a shared communications bus through interface modules. An initiator functional block may communicate with a shared communications bus through an initiator interface module and a target functional block may communicate with a shared communications bus through a target interface module.

[0042] An initiator interface module issues and receives read and write requests to and from functional blocks other than the one with which it is associated. In one embodiment, an initiator interface module is typically connected to a CPU, a digital signal processing (DSP) core, or a direct memory access (DMA) engine.

[0043]FIG. 2 is a block diagram of an embodiment of an initiator interface module 800. Initiator interface module 800 includes clock generator 802, data flow block 806, arbitrator block 804, address/command decode block 808, configuration registers 810, and synchronizer 812. Initiator interface module 800 is connected to a shared communications bus 814 and to an initiator functional block 816. In one embodiment, shared communications bus 814 is a shared communications bus that connects sub-systems, as bus 112 does in FIG. 1.

[0044] Clock generator 802 is used to perform clock division when initiator functional block 816 runs synchronously with respect to shared communications bus 814 but at a different frequencies. When initiator functional block 816 runs asynchronously with respect to communications bus 814, clock generator 802 is not used, but synchronizer 812 is used. Arbitrator block 804 performs arbitration for access to shared communications bus 814. In one embodiment, a multi-level arbitration scheme is used wherein arbitrator module 804 includes logic circuits that manage pre-allocated bandwidth aspects of first level arbitration and also logic that manages second level arbitration. Data flow block 806 includes data flow first-in first-out (FIFO) buffers between shared communications bus 814 and initiator functional block 816, in addition to control logic associated with managing a transaction between shared communications bus 814 and initiator functional block 816. The FIFO buffers stage both the address and data bits transferred between shared communications bus 814 and initiator functional block 816. In one embodiment, shared communications bus 814 implements a memory mapped protocol. Specific details of an underlying computer bus protocol are not significant to the invention, provided that the underlying computer bus protocol supports some operation concurrency. A preferred embodiment of a bus protocol for use with the present invention is one that supports retry transactions or split transactions, because these protocols provide a mechanism to deliver operation concurrency by interrupting a multi-cycle transaction to allow transfers belonging to other unrelated transactions to take place. These protocols allow for higher transfer efficiencies because independent transactions may use the bus while an initiator waits for a long latency target to return data that has been previously requested by an initiator.

[0045] Address/command decode block 808 decodes an address on shared communications bus 814 to determine if a write is to be performed to registers associated with initiator functional block 816. Address/command decode block 808 also decodes incoming commands. Configuration registers 810 store bits that determine the state of module 800, including bandwidth allocation and client address base. One register 810 stores an identification (ID) which is a set of bits uniquely identifying initiator functional block 816.

[0046]FIG. 3 is a block diagram of an embodiment of a target interface module 900. Target interface module 900 is connected to shared communications bus 914 and to target functional block 918. Target interface module 900 includes clock generator 902, data flow block 906, address/command decode block 908, synchronizer 912, and state registers in state control block 916. Blocks of target interface module 900 that are named similarly to blocks of initiator module 800 function in substantially the same way as explained with respect to initiator block 800. State registers and state control block 916 include registers that store, for example, client address base and an identifier for target functional block 918.

[0047] In one embodiment, an initiator functional block such as initiator functional block 816 may also act as a target functional block in that it has the capability to respond to signals from other functional blocks or sub-systems as well as to initiate actions by sending signals to other functional blocks or sub-systems.

[0048]FIG. 4 is a block diagram of a part of a computer system 1000 according to one embodiment. FIG. 4 is useful in illustrating multilevel connection identification. System 1000 includes initiator functional block 1002, which is connected to initiator interface module 1004 by interconnect 1010. Initiator interface module 1004 is connected to target interface module 1006 by shared communications bus 1012. Target interface module 1006 is connected to target functional block 1008 by an interconnect 1010. Typically, shared communications bus 1012 is analogous to shared communications bus 112 of FIG. 1 or to shared communications bus 114 of FIG. 1. Interconnects 1010 are typically analogous to interconnect 115 of FIG. 1 in that they connect functional blocks to interface modules and are point-to-point, rather than shared, interconnects. Interconnects 1010 are typically physically shorter than shared communications bus 1012 because of their local nature. As will be explained more fully below, system 1000 uses two different levels of connection identification depending upon the requirements of a particular functional block. “Global” connection identification information is sent on shared communications bus 1012, while “local” connection information, or thread identification information, is sent in interconnects 1010.

[0049]FIG. 5 is a block diagram of one embodiment of a shared communications bus 1012. Shared communications bus 1012 is shown connected to entities A, B, C, D and E, which may be interface modules or functional blocks. Shared communications bus 1012 is composed of a set of wires. Data wires 230 provide direct, high efficiency transport of data traffic between functional blocks on shared communications bus 1012. In one embodiment, shared communications bus 1012 supports a bus protocol that is a framed, time division multiplexed, fully pipelined, fixed latency communication protocol using separate address, data and connection identification wires. The bus protocol supports fine grained interleaving of transfers to enable high operation concurrency, and uses retry transactions to efficiently implement read transactions from target devices with long or variable latency. Details of the arbitration method used to access shared communications bus 1012 are not required to understand the present invention. The delay from when an initiator functional block drives the command and address until the target functional block drives the response is known as the latency of shared communications bus 1012. The bus protocol supports arbitration among many initiator functional blocks and target functional blocks for access to the bus. In the embodiment shown, arbitration for access to shared communications bus 1012 is performed by an initiator interface module, such as module 1004 of FIG. 4. In other embodiments, arbitration is performed by functional blocks directly, or by a combination of interface modules and functional blocks. In one embodiment, a bus grant lasts for one pipelined bus cycle. The protocol does not forbid a single functional block from becoming a bus owner for consecutive bus cycles, but does require that the functional block successfully win arbitration on consecutive cycles to earn the right.

[0050] Shared communications bus 1012 includes separate address, data, and control wires. Other embodiments may include multiplexed address, data, and control signals that share a wire or wires. Such an embodiment would provide high per-wire transfer efficiency because wires are shared among address and data transfers. A non-multiplexed embodiment of shared communications bus 1012 may be more appropriate for communication between functional blocks on a single integrated circuit chip because wires are relatively inexpensive and performance requirements are usually higher on a single integrated circuit chip.

[0051] Clock line 220 is a global signal wire that provides a time reference signal to which all other shared communications bus 1012 signals are synchronized. Reset line 222 is a global signal wire that forces each connected functional block into a default state from which system configuration may begin. Command line 224 carries a multi-bit signal driven by an initiator bus owner. In various embodiments, the multi-bit command signal may convey various types of information. For example, a command signal may indicate a transfer type, information regarding duration of a connection, and expected initiator and target behavior during the connection. In one embodiment, the command signal includes one or more bits indicating the beginning and end of a connection. In one embodiment, for example, one bit may indicate the status of a connection. If the bit is zero, the current transfer is the final transfer in the connection. After the receipt of a zero connection status bit, the next receipt of a connection status bit that is a logic one indicates that the transfer is the first in a newly opened connection. Each subsequently received one connection status bit then indicates that the connection is still open.

[0052] Supported transfer types in this embodiment include, but are not limited to read and write transfers. Address lines 228 carry a multi-bit signal driven by an initiator bus owner to specify the address of the object to be read or written during the current transfer. Response lines 232 carry a multi-bit signal driven by a target to indicate the status of the current transfer. Supported responses include, but are not limited to the following responses. A NULL response indicates that the current transfer is to be aborted, presumably because the address does not select any target. A data valid and accepted (DVA) response indicates, in the case of a read, that the target is returning requested data on data lines 230. In the case of a write, a DVA response indicates that the target is accepting the provided data from data lines 230. A BUSY response indicates that the selected target has a resource conflict and cannot service the current request. In this case an initiator should reattempt the transfer again later. A RETRY response indicates that the selected target could not deliver the requested read data in time, but promises to do so at a later time. In this case an initiator must reattempt the transfer at a later time.

[0053] Connection identifier (CONNID) lines 226 carry a multi-bit signal driven by an initiator bus owner to indicate which connection the current transfer is part of. A connection is a logical state, established by an initiator, in which data may pass between the initiator and an associated target. The CONNID typically transmits information including the identity of the functional block initiating the transfer and ordering information regarding an order in which the transfer must be processed. In one embodiment, the information conveyed by the CONNID includes information regarding the priority of the transfer with respect to other transfers. In one embodiment the CONNID is a eight-bit code. An initiator interface module sends a unique CONNID along with an initial address transfer of a connection. Later transfers associated with this connection (for example, data transfers) also provide the CONNID value so both sender and receiver (as well as any device monitoring transfers on shared communications bus 1012) can unambiguously identify transfers associated with the connection. One advantage of using a CONNID is that transfers belonging to different transactions can be interleaved arbitrarily between multiple devices on a per cycle basis. In one embodiment, shared communications bus 1012 implements a fully pipelined protocol that requires strict control over transaction ordering in order to guarantee proper system operation. Without the use of a CONNID, ordering constraints within a particular transaction may be violated because transfers associated with a particular connection are not identified.

[0054] Because a first command may be rejected by a BUSY response while a later command is already in flight, it is essential to provide mechanisms that allow full control over which commands complete. If such control is not present, ambiguous system behavior can result. For instance, if a single initiator interface module issues a sequence of dependent read and write commands, a busy response to one of the commands could result in later commands returning the wrong data. One solution to such problems is to avoid overlapping dependent commands. This solution, however, increases the latency of every dependent command in order to ensure proper results. The present invention uses a CONNID signal, in part, to allow overlapping of dependent commands. Therefore, use of a CONNID improves system performance and efficiency. Another benefit of the CONNID of the present invention is that communication system predictability is enhanced because it allows a shared functional block to respond to requests based upon quality of service guarantees that may vary between connections. For example, data requested to operate a computer display cannot tolerate unpredictable delay because delay causes the display to flicker. Therefore, the CONNID may be used to prioritize data requests from a display controller so that requests from the display controller to a common resource are serviced before other requests. The present invention also allows for flexible reconfiguration of the CONNID to retune system performance.

[0055]FIG. 6 is a timing diagram of a pipelined write transaction consisting of two write transfers on shared communications bus 1012. Reference may also be made to FIG. 5. A single pipelined bus transfer, as shown in FIG. 6, includes an arbitration cycle (not shown), followed by a command/address/CONNID (CMD 324/ADDR 328/CONNID 326) cycle (referred to as a request, or REQ cycle), and completed by a DATA 330/RESP 342 cycle (referred to as a response, or RESP cycle). In one embodiment, the number of cycles between a REQ cycle and a RESP cycle is chosen at system implementation time based upon the operating frequency and module latencies to optimize system performance. The REQ-RESP latency, in one embodiment, is two cycles and is labeled above the DATA 330 signal line on FIG. 6. Therefore, a complete transfer time includes four shared communications bus 1012 cycles, arbitration, request, delay and response.

[0056] Two transfers are shown in FIG. 6. On cycle 1, initiator E drives REQ fields 340 to request a WRITE transfer to address ADDRE0. This process is referred to as issuing the transfer request. In one embodiment, a single target is selected to receive the write data by decoding an external address portion of ADDRE0. On cycle 3 (a REQ-RESP latency later), initiator E drives write data DATAE0 on the DATA wires; simultaneously, the selected target A drives RESP wires 342 with the DVA code, indicating that A accepts the write data. By the end of cycle 3, target A has acquired the write data, and initiator E detects that target A was able to accept the write data; and the transfer has thus completed successfully.

[0057] Meanwhile (i.e. still in cycle 3), initiator E issues a pipelined WRITE transfer (address ADDRE1) to target A. The write data and target response for this transfer both occur on cycle 5, where the transfer completes successfully. Proper operation of many systems and sub-systems rely on the proper ordering of related transfers. Thus, proper system operation may require that the cycle 3 WRITE complete after the cycle 1 WRITE transfer. In FIG. 6, the CONNID field conveys crucial information about the origin of the transfer that can be used to enforce proper ordering. A preferred embodiment of ordering restrictions is that the initiator and target collaborate to ensure proper ordering, even during pipelined transfers. This is important, because transfer pipelining reduces the total latency of a set of transfers (perhaps a single transaction), thus improving system performance (by reducing latency and increasing usable bandwidth).

[0058] According to the algorithm of one embodiment:

[0059] 1. An initiator may issue a transfer Y:

[0060] a) if transfer Y is the oldest, non-Issued, non-retired transfer among the set of transfer requests it has with matching CONNID, or

[0061] b) if all of the older non-retired transfers with matching CONNID are currently issued to the same target as transfer Y. If issued under this provision, transfer Y is considered pipelined with the older non-retired transfers.

[0062] 2. A target that responds to a transfer X in such a way that the initiator might not retire the transfer must respond BUSY to all later transfers with the same CONNID as transfer X that are pipelined with X.

[0063] Note that an older transfer Y that is issued after a newer transfer X with matching CONNID is not considered pipelined with X, even if Y Issues before X completes. This situation is illustrated in FIG. 7. If target A has a resource conflict that temporarily prevents it from accepting DATAEO associated with the WRITE ADDRE0 from cycle 1, then A responds BUSY. Step 2 of the foregoing algorithm requires that A also reject (using BUSY) any other pipelined transfers from the same CONNID (in this case, CONNID 1), since the initiator cannot possibly know about the resource conflict until after the REQ-RESP latency has passed. Thus, target A must BUSY the WRITE ADDRE1 that is issued in cycle 3, because it has the same CONNID and was issued before the initiator could interpret the BUSY response to the first write transfer, and is thus a pipelined transfer. Furthermore, the second attempt (issued in cycle 4) of the WRITE ADDRE0 transfer is allowed to complete because it is not a pipelined transfer, even though it overlaps the cycle 3 WRITE ADDRE1 transfer.

[0064] Note that target A determines that the cycle 4 write is not pipelined with any earlier transfers because of when it occurs and which CONNID it presents, and not because of either the CMD nor the ADDR values. Step 1 of the algorithm guarantees that an initiator will only issue a transfer that is the oldest non-issued, non-retired transfer within a given connection. Thus, once the first WRITE ADDRE0 receives the BUSY response in cycle 3, it is no longer issued, and so it becomes the only CONNID=1 transfer eligible for issue. It is therefore impossible for a properly operating initiator to issue a pipelined transfer in cycle 4, given that an initial cycle 1 transfer received a BUSY response and the REQ-RESP latency is two cycles.

[0065] One embodiment of the initiator maintains a time-ordered queue consisting of the desired transfers within a given CONNID. Each transfer is marked as non-issued and non-retired as they are entered into the queue. It is further marked as pipelined if the immediately older entry in the queue is non-retired and addresses the same target; otherwise, the new transfer is marked non-pipelined. Each time a transfer issues it is marked as issued. When a transfer completes (i.e., when the RESP cycle is finished) the transfer is marked non-issued. If the transfer completes successfully, it is marked as retired and may be deleted from the queue. If the transfer does not complete successfully, it will typically be re-attempted, and thus can go back into arbitration for re-issue. If the transfer does not complete successfully, and it will not be re-attempted, then it should not be marked as retired until the next transfer, if it exists, is not marked as issued. This restriction prevents the initiator logic from issuing out of order. As the oldest non-Retired transfer issues, it is marked as issued. This allows the second-oldest non-retired transfer to arbitrate to issue until the older transfer completes (and is thus marked as non-issued), if it is marked as pipelined.

[0066] An embodiment of the target implementation maintains a time-ordered queue whose depth matches the REQ-RESP latency. The queue operates off of the bus clock, and the oldest entry in the queue is retired on each bus cycle; simultaneously, a new entry is added to the queue on each bus cycle. The CONNID from the current REQ phase is copied into the new queue entry. In addition, if the current REQ phase contains a valid transfer that selects the target (via the External Address), then “first” and “busy” fields in the new queue entry may be set; otherwise, the first and busy bits are cleared. The first bit will be set if the current transfer will receive a BUSY response (due to a resource conflict) and no earlier transfer in the queue has the same CONNID and has its first bit set. The first bit implies that the current transfer is the first of a set of potentially-pipelined transfers that will need to be BUSY'd to enforce ordering. The busy bit is set if either the target has a resource conflict or one of the earlier transfers in the queue has the same CONNID and has the first bit set. This logic enforces the REQ-RESP pipeline latency, ensuring that the target accepts no pipelined transfers until the initiator can react to the BUSY response to the transfer marked first.

[0067] Application of the algorithm to the initiators and targets in the communication system provides the ability to pipeline transfers (which increases per-connection bandwidth and reduces total transaction latency) while maintaining transaction ordering. The algorithm therefore facilitates high per-connection performance. The fundamental interleaved structure of the pipelined bus allows for high system performance, because multiple logical transactions may overlap one another, thus allowing sustained system bandwidth that exceeds the peak per-connection bandwidths. For instance, FIG. 8 demonstrates a system configuration in which initiator E needs to transfer data to target A on every other bus cycle, while initiator D requests data from target B on every other bus cycle. Since the communication system supports fine interleaving (per bus cycle), the transactions are composed of individual transfers that issue at the natural data rate of the functional blocks; this reduces buffering requirements in the functional blocks, and thus reduces system cost. The total system bandwidth in this example is twice the peak bandwidth of any of the functional blocks, and thus high system performance is realized.

[0068] The present invention adds additional system-level improvements in the area of efficiency and predictability. First, the connection identifier allows the target to be selective in which requests it must reject to preserve in-order operation. The system only need guarantee ordering among transfers with the same CONNID, so the target must reject (using BUSY) only pipelined transfers. This means that the target may accept transfers presented with other CONNID values even while rejecting a particular CONNID. This situation is presented in FIG. 9, which adds an interleaved read transfer from initiator D to the pipelined write transfer of FIG. 7. All four transfers in FIG. 9 select target A, and A has a resource conflict that prevents successful completion of the WRITE ADDRE0 that issues in cycle 1. While the rejection of the first write prevents A from accepting any other transfers from CONNID 1 until cycle 4, A may accept the unrelated READ ADDRD0 request of cycle 2 if A has sufficient resources. Thus, overall system efficiency is increased, since fewer bus cycles are wasted (as would be the case if target A could not distinguish between connections).

[0069] Second, in one embodiment the connection identifier allows the target to choose which requests it rejects. The target may associate meanings such as transfer priority to the CONNID values, and therefore decide which requests to act upon based upon a combination of the CONNID value and the internal state of the target. For instance, a target might have separate queues for storing transfer requests of different priorities. Referring to FIG. 9, the target might have a queue for low priority requests (which present with an odd CONNID) and a queue for high priority requests (which present with an even CONNID). Thus, the CONNID 1 WRITE ADDRE0 request of cycle 1 would be rejected if the low-priority queue were full, whereas the CONNID 2 READ ADDRD0 transfer could be completed successfully based upon available high-priority queue resources. Such differences in transfer priorities are very common in highly-integrated electronic systems, and the ability for the target to deliver higher quality of service to higher priority transfer requests adds significantly to the overall predictability of the system.

[0070] As FIG. 9 implies, the algorithm described above allows a target to actively satisfy transfer requests from multiple CONNID values at the same time. Thus, there may be multiple logical transactions in flight to and/or from the same target, provided that they have separate CONNID values. Thus, the present invention supports multiple connections per target functional block.

[0071] Additionally, an initiator may require the ability to present multiple transactions to the communications system at the same time. Such a capability is very useful for initiator such as direct memory access (DMA) devices, which transfer data between two targets. In such an application, the DMA initiator would present a read transaction using a first CONNID to a first target that is the source of the data, and furthermore present a write transaction using a second CONNID to a second target that is the data destination. At the transfer level, the read and write transfers could be interleaved. This reduces the amount of data storage in the DMA initiator, thus reducing system cost. Such an arrangement is shown in FIG. 10, where initiator E interleaves pipelined read transfers from target A with pipelined write transfers to target B. Thus, the present invention supports multiple connections per initiator functional block.

[0072] The control structures required to support implementation of the present invention, as described above with respect to the algorithm, are simple and require much less area than the data buffering area associated with traditional protocols that do not provide efficient fine interleaving of transfers. Thus, the present invention minimizes communication system area and complexity, while delivering high performance and flexibility.

[0073] Finally, the CONNID values that are associated with particular initiator transactions should typically be chosen to provide useful information such as transfer priorities but also to minimize implementation cost. It is useful to choose the specific CONNID values at system design time, so the values can be guaranteed to be unique and can be ordered to simplify comparison and other operations. Furthermore, it is frequently useful to be able to change the CONNID values during operation of the communications system so as to alter the performance and predictability aspects of the system. Preferred implementations of the present invention enable flexible system configuration by storing the CONNID values in ROM or RAM resources of the functional blocks, so they may be readily reconfigured at either system build time or system run time.

[0074]FIG. 11 shows an interconnect 1010, which is a point-to-point interconnect as shown in FIG. 4. Interconnect 1010 includes additional signals as compared to the protocol described with reference to FIG. 5. As will be explained below, some of the additional signals are particularly useful as signals sent over point-to-point interconnects such as interconnects 1010. The protocol of interconnect 1010 controls point-to-point transfers between a master entity 1102 and a slave entity 1104 over a dedicated (non-shared) interconnect. Referring to FIG. 5, a master entity may be, for example, initiator functional block 1002 or target interface module 1006. A slave entity may be, for example, initiator interface module 1004 or target functional block 1008.

[0075] Signals shown in FIG. 11 are labeled with signal names. In addition, some signal names are followed by a notation or notations in parentheses or brackets. The notations are as follows:

[0076] (I) The signal is optional and is independently configurable

[0077] (A) The signal must be configured together with signals having similar notations

[0078] (AI) The signal is independently configurable if (A) interface modules exist

[0079] [#] Maximum signal width

[0080] The clock signal is the clock of a connected functional block. The command (Cmd) signal indicates the type of transfer on the bus. Commands can be issued independent of data. The address (Addr) signal is typically an indication of a particular resource that an initiator functional block wishes to access. Request Accept (ReqAccept) is a handshake signal whereby slave 1104 allows master 1102 to release Cmd, Addr and DataOut from one transfer and reuse them for another transfer. If slave 1104 is busy and cannot participate in a requested transfer, master 1102 must continue to present Cmd, Addr and DataOut. DataOut is data sent from a master to a slave, typically in a write transfer. DataIn typically carries read data.

[0081] Response (Resp) and DataIn are signals sent from slave 1104 to master 1102. Resp indicates that a transfer request that was received by slave 1104 has been serviced. Response accept (RespAccept) is a handshake signal used to indicate whether the master allows the slave to release Resp and DataIn.

[0082] Signals Clock, Cmd, Addr, DataOut, ReqAccept, Resp, DataIn, and RespAccept, in one embodiment, make up a basic set of interface module signals. For some functional blocks, the basic set may be adequate for communication purposes.

[0083] In other embodiments, some or all of the remaining signals of bus 1012 may be used. In one embodiment, Width is a three-bit signal that indicates a width of a transfer and is useful in a connection that includes transfers of variable width. Burst is a multibit signal that allow individual commands to be associated within a connection. Burst provides an indication of the nature of future transfers, such as how many there will be and any address patterns to be expected. Burst has a standard end marker. Some bits of the Burst field are reserved for user-defined fields, so that a connection may be ignorant of some specific protocol details within a connection.

[0084] Interrupt and error signals are an important part of most computer systems. Interrupt and error signals generated by initiator or target functional blocks are shown, but the description of their functionality is dependent upon the nature of a particular functional block and is not important to understanding the invention.

[0085] Request Thread Identifier (ReqThreadID), in one embodiment, is a four-bit signal that provides the thread number associated with a current transaction intended for slave 1104. All commands executed with a particular thread ID must execute in order with respect to one another, but they may execute out of order with respect to commands from other threads. Response Thread Identifier (RespThreadID) provides a thread number associated with a current response. Because responses in a thread may return out of order with respect to other threads, RespThreadID is necessary to identify which thread's command is being responded to. In one embodiment, ReqThreadID and RespThreadID are optional signals, but if one is used, both should be used.

[0086] Request Connection Identifier (ReqConnID) provides the CONNID associated with the current transaction intended for the target. CONNIDs provide a mechanism by which a system entity may associate particular transactions with the system entity. One use of the CONNID is in establishing request priority among various initiators. Another use is in associating actions or data transfers with initiator identity rather than the address presented with the transaction request.

[0087] Request Thread Busy (ReqThreadBusy) allows the slave to indicate to the master that it cannot take any new requests associated with certain threads. In one embodiment, the ReqThreadBusy signal is a vector having one signal per thread, and a signal asserted indicates that the associated thread is busy.

[0088] Response Thread Busy (RespThreadBusy) allows the master to indicate to the slave that it cannot take any responses (e.g., on reads) associated with certain threads. In one embodiment, the RespThreadBusy signal is a vector having one signal per thread, and a signal asserted indicates that the associated thread is busy.

[0089] The ReqThreadBusy and RespThreadBusy signals described above are an example of a per-thread flow control mechanism. Other per-thread flow control mechanisms exist. Alternately a credit-based mechanism may be used. In a credit-based mechanism, the slave informs the master how many or approximately how many transfers it can accept. In one embodiment, this information is communicated by the slave raising a credit signal for a single cycle for each transfer it can accept. Thus, the number of “credits” or transfers the slave can accept is communicated by the number of cycles that the signal is active.

[0090] A timing diagram illustrating one embodiment of a credit-based per-thread flow control mechanism is illustrated in FIG. 13. Commands from a master device are reflected by active high signals on CMD line 1310. Credit line 1320 reflects active high signals issued by the slave device that indicate how many commands it can accept. Thus, as illustrated, two credits indicated by two clock pulses 1330 are initially available. One credit is used by issuance of a command 1335. Then, a new credit is issued 1340. Two commands are issued 1345, resulting in no availability of credits. No subsequent commands are issued until the credit line has issued further credits. Thus, after a new credit appears 1350 on credit line 1320 the master device can issue a command 1355.

[0091] Alternately, a count of the number of credits may be sent each cycle, or during one or a predetermined number of cycles. In this embodiment, it is contemplated that multiple signal lines would be required to communicate the count.

[0092] Utilizing per-thread flow control prevents the interface that is shared among a plurality of slave and master devices from blocking. When several threads are sharing an interconnect, it is generally desirable that individual threads remain independent of one another. In order to maintain independent operation, interfaces into the interconnect should not be allowed to block. For example, if the master were to send a transfer which cannot be accepted by the slave, the slave would not assert ReqAccept, thus forcing the master to hold the transfer on the interface. This in turn would prevent any other thread from presenting a transfer on the interface, i.e. the interconnect would be effectively blocked for all threads. By having individual per-thread flow control, the master can know not to send a transfer on a particular thread if the slave has indicated using the ThreadBusy signals that it cannot accept transfers on that thread.

[0093] In some embodiments, there may be a need to specify as to when ThreadBusy is presented following a transfer that requires a flow control action by the slave, and/or when the master cannot send a new transfer following the assertion of ThreadBusy. For example, it may take the slave a few clock cycles to determine that its resources are depleted or about to be depleted (therefore necessitating the assertion of ThreadBusy). In another example, it may take the master a few dock cycles to react to the assertion of the ThreadBusy signal. During those clock cycles the master may have issued additional commands (e.g., data transfer). One way to ensure that the interface does not block is to specify the timing of the ThreadBusy signals with respect to the transfers for both master and slave, and provide sufficient buffering in the slave to absorb any new transfers of commands by the master after the ThreadBusy signal has been asserted by the slave.

[0094] Provision of per-thread flow control permits end-to-end performance guarantees to be determined across the interconnect. End-to-end data channels are linked using the threads of the interconnect, and per-thread flow control allows for end-to-end flow control between a particular initiator and target. One embodiment of end-to-end performance guarantees is described with reference to FIGS. 14, 15a, 15 b, 16, 17, 18, 19, 20 and 21.

[0095] An exemplary system is shown in FIG. 14. The system consists of one or more functional blocks 1405, 1410, 1415, 1420, 1425, 1430 and one or more communication subsystems 1435, 1440 that are all tied together using a set of interfaces 1445, 1450, 1455, 1460, 1465, 1470, 1475, 1480. The functional blocks 145, 1410, 1415, 1420, 1425, 1430 perform the actual work done by the system (such as computation). In this document, the focus is on communication between functional blocks. Functional blocks are broken down into initiators 145, 1410, 1420 and targets 1415, 1425, 1430, depending on whether a functional block initiates or is the target of a data flow.

[0096] For purpose of discussion herein, a data flow is a set of commands, data items or events being communicated over time between two functional blocks. Any given request from an initiator to a target is a request to perform some action. For example, read and write requests cause a target to retrieve and store the data respectively. A channel is a portion of a set of physical resources in a component that is dedicated to service a particular data flow. For example, a target may have different data channels to service different incoming data flows. A communication subsystem transmits data items from one of its input ports to an output port, using a dedicated data channel for each data flow. Each of the ports leads from/to another communications subsystem or a functional block via an interface. Interfaces also have dedicated channels for each data flow.

[0097] Thus, for example, the end-to-end progress of a data flow may be as follows. The data flow originates at an initiator. It travels to an input port of a first communication subsystem via a channel of an interface. Once inside the first communication subsystem, it travels to an output port, via a channel. It then may cross another interface to other communication subsystems until it finally reaches a target. It is serviced by a dedicated channel inside the target and completes if no response is needed. If a response is needed, the data flow then reverses its direction through the communications subsystem(s), and completes when it reaches the originating initiator.

[0098] In one embodiment, the interfaces tying the different components together have the ability to tag each request and response transmitted with the identity of the channel it belongs to, and the ability to transmit flow control on a per-channel basis. These features of the interfaces are desirable to achieve independent data channels that allow the quality of service guarantees given by an individual component to be propagated to another component that it is connected to via an interface. Thus, for example with reference to FIG. 4, the quality of service guarantees include the signal propagation time from initiator functional block 1002 to target functional block 1008.

[0099]FIG. 15a shows an example of a functional block's quality of service guarantees. In this case, a memory system is shown that has differential quality of service guarantees for three channels. The quality of service guarantees given are minimum bandwidth and maximum latency while servicing an 8-byte read or write request. In this example, each channel can sustain 10 M 8-byte requests per second. Channel 1 has been optimized for lower latency and guarantees a maximum service latency of 100 ns, while channels 2 and 3 only guarantee a maximum service latency of 200 ns. It is common for the performance guarantees given by a particular functional block to be much more complex than shown here and to depend on a lot of different factors such as request size, inter-arrival interval, etc. The metrics used here have been chosen to keep the example simple. While maximum latency and minimum bandwidth are two common types of performance parameters, quality of service guarantees using other parameters such as, maximum outstanding requests, maximum variance in service latency, etc. are equally possible. In addition, if a particular component is too complex to easily achieve differential quality of service guarantees, the methodology can be applied recursively to that component by breaking it down into smaller, more manageable pieces.

[0100]FIG. 15b shows an example of a communication subsystem's quality of service guarantees. This particular communication system has two ports for connecting initiators X 210 and Y 215 and one port for connecting a target Z 220. Port X 210 in turn supports two different incoming channels, port Y one incoming channel and port Z, three outgoing channels. This yields three data channels (A, B, and C) within the communications subsystem. In the present example, no quality of service guarantees have been given for channel A, again to simplify the discussion herein and also to note that sometimes certain data flows do not have guarantees. Channel B is serviced with a minimum bandwidth of 4 bytes every 4 cycles and a maximum latency of 3 cycles for each direction. Channel C is serviced with a minimum bandwidth of 4 bytes every 2 cycles and a maximum latency of 5 cycles for each direction.

[0101] A variety of techniques may be used to determine component guarantees. One technique that can be used to determine quality of service guarantees for a particular component such as a communication subsystem is described in U.S. Pat. No. 5,948,089, which is herein incorporated by reference.

[0102] In one embodiment, the resultant guarantees may be valid under all possible system conditions, i.e. the ability of the system to meet its performance requirements is given by design. In addition, in one embodiment, analysis is greatly simplified by making each of the data flows independent of one another, thus breaking the complex system analysis problem into a set of simpler data flow analysis problems.

[0103] As shown in FIG. 14, interfaces 1445, 1450, 1455, 1460, 1465, 1470, 1475, 1480 are used to tie components (both functional blocks and communications subsystems) together. In order to allow for end-to-end performance guarantees, these interfaces must be able to propagate multiple individual data flows with individual flow control on each channel. An example interface is shown in FIG. 16. Here the initiator 1605 sends a request 1620, potentially with write data 1625,to the target 1610. The target 1610 replies with a response 1630 and possibly also with read data 1635. In order to allow multiple independent data flows to cross this interface, a channel identifier 1615 is sent from the initiator 1605 to the target 1610 with each request, a channel identifier 1632 is sent from the target to the initiator 1605 with each response, and a per-data-flow flow control signal 1640 is sent from target to initiator. An example of such an interface is described in PCT/US99/26901, filed Nov. 12, 1999.

[0104] One embodiment of a process for calculating end-to-end system performance guarantees for different data flows is shown in FIG. 17. For each initiator data flow in the system 1710, a mapping of the data flow through the channels of the system is first determined 1715. An exemplary mapping process is further illustrated in FIG. 18 and will be discussed below.

[0105] Next, referring to FIG. 17, the parameters of the quality of service guarantees of the data channels of the components involved in this data flow are aligned to be uniform step 1720. It is preferred that all guarantees be uniform in order to facilitate a global aggregation at step 1730. Since different components are developed by different people and/or companies it is rare that all quality of service guarantees on all subsystems are expressed in the same units. More likely, the units have to be converted, time scales translated, or the desired quality of service guarantees derived from those given. For example, one component may specify a bandwidth guarantee in terms of Mbytes/s whereas another may specify a number of transfers/s irrespective of transfer size. This aligning process may in turn put requirements on the type of guarantees that individual components should give, in order to accomplish the aggregation process.

[0106] Once the units have been aligned along the entire data flow, the aggregate quality of service guarantees for that data flow through the entire system can be calculated step 1730. The aggregation mechanism used depends on the type of parameter used to express the quality of service guarantees. For example, maximum latencies for each component are added together to give a global maximum latency. In contrast, the minimum component bandwidth guarantee along the entire path determines the minimum bandwidth service guarantee.

[0107]FIG. 18 focuses in on one embodiment of the data flow mapping process that is part of the global mechanism shown in FIG. 17. For each component in a path from initiator to target, e.g., through several interfaces and one or more communication subsystems step 1710, an available channel is chosen to carry the initiator channel step 1715. Picking a particular data flow out of the set of available channels for a given component can be a complex task. Many possible ways of accomplishing the task and optimizing the overall system performance exist. For example, one could choose the channel that most closely matches the performance requirements of the initiator's data flow. It is also possible to split a data flow among multiple paths from initiator to target. In the present example, it is simply assumed that a valid mapping is chosen.

[0108] Once a channel is used in a given component, it is marked as used, step 1820, so that it will not be chosen for another initiator data flow. The algorithm continues until the entire data flow has been mapped from initiator to target. Note that a given component may have more channels than are required in this particular system. This is not a problem and the leftover channels are simply not used in that component. Conversely, if a component does not have enough channels to support the system, that component is not appropriate for the system and a different component should be chosen.

[0109] In addition, a component may allow multiple data flows to share a channel. In this case, the performance guarantees for a particular data flow may well depend on the characteristics of the other data flow(s), and can only be well-bounded if the initiator(s) of the other data flow(s) give guarantees about the characteristics of those data flows (such as maximum issue bandwidth).

[0110] The mapping of data flows to channels need not be a static mapping that is fixed in a system. The system may well be reconfigurable in order to adapt to different communication patterns. For example, a communication subsystem can reallocate bandwidth resources from one data channel to another to accommodate shifting data flow quality of service requirements.

[0111] In one embodiment, the system is loaded with a revised mapping and the system is reconfigured accordingly. This may be performed while the system is operating, for example, in a real time manner.

[0112] In another embodiment, reconfiguration of the system may be accomplished by the system itself providing even further flexibility and adaptability. In some embodiments, this may also be performed in a real time manner further enhancing the functionality of the system.

[0113] In one embodiment, the system is operating using at least one mapping of a data flow. In one embodiment, the aggregation of the guarantees of the selected data channels meet the application requirements. However, it is contemplated that, in one embodiment, the system may initially be operated using a first mapping that generates an aggregation that does not meet the desired guarantees. However, the system would perform the steps described herein to determine a second mapping of data flows that results in aggregate guarantees that meets the application requirements. The system may then be reconfigured according to the second mapping of data flows that meets the application requirements.

[0114] A system may also be configurable to be adaptable to different mappings that meet different application requirements, the mapping chosen based on determined criteria. Thus a system may be able to provide multiple mappings that meet the desired requirements. The particular mapping utilized may then be selected according to other criteria that may or may not relate to the desired requirements.

[0115] In some embodiments, components of a system may be replaced with other components of a different internal implementation structure. As long as the quality of service guarantees of the new components are identical to those of the replaced components, the system-wide end-to-end guarantees remain intact. Thus, such an exchange can remain transparent to the other components of the system.

[0116]FIG. 19 is an example that shows how a unit translation progresses for the example functional block and communication subsystem shown in FIGS. 15a and 15 b. It is assumed that the desired units are Mbytes/s for minimum bandwidth guarantees. In the present embodiment, for both the memory system and the communication subsystem, the quality of service guarantees are dependent on the size of requests issued by a particular initiator. In FIG. 19, the bandwidth guarantees of the two components for the request sizes of 4, 8 and 16 bytes are aligned. Note that the original memory system bandwidth guarantees were stated in terms of 8-byte requests per second. When smaller requests are used in the present embodiment it is conservatively assumed that the service rate does not increase. That is why the bandwidth is lower for the smaller requests.

[0117] In the communications subsystem, bandwidth guarantees were given for 4-byte requests and it is assumed that an 8 or 16-byte request can be broken down into 2 or 4 4-byte requests respectively. However, the units chosen were cycles, so the communication subsystem's operating frequency also needs to be known in order to align the bandwidth guarantees. Assuming that a 100 MHz frequency is chosen for the communications subsystem, i.e. that its cycle time is 10 ns, the resulting minimum bandwidth guarantees are 100 Mbytes/s for all request sizes of 4 bytes or above.

[0118] An example application of the quality of service aggregation mechanism using a simple system is shown in FIG. 20. An initiator A 2005 sources two data flows shown as 1 2010 and 2 2015. A second initiator B sources only one data flow 2025. The communications subsystem 2030 receives the initiator data flows on ports X 2035 and Y 2040, respectively, and sends all of these flows to a target C 2045 via port Z 2050.

[0119] The first table in FIG. 21 shows one embodiment of the mapping of each initiator data flow though the system. So, for example, initiator A data flow 1 2010 is sent to communications subsystem port X 2035 via channel A 2060 of interface 1. It traverses the communication subsystem 2030 via channel B 2037, exits via port Z 2065, and is sent to target C 2045 via interface 3 channel A 2070. At target C 2045 channel 1 2075 is chosen to handle this data flow.

[0120] The second table of FIG. 21 shows an example of quality of service guarantees after unit alignment for the three different initiator data flows. In this example, it is assumed that initiator A flow 1 is using 4-byte requests, initiator A flow 2 is using 16-byte requests, and initiator B flow 1 is using 8-byte requests. The unit conversion and alignment is accomplished using the process described above.

[0121] The last table of FIG. 21 shows the exemplary system end-to-end quality of service guarantees for the three initiator data flows. In order to calculate the minimum bandwidth in the system for each data flow, the minimum bandwidth of each mapped data flow through each component is found. Maximum latencies are found by adding each of the component latencies along a data flow from initiator to target.

[0122] The Request Connection Identifier (ReqConnID) provides the CONNID associated with the current transaction intended for slave or target. CONNIDs provide a mechanism by which a system entity may associate particular transactions with the system entity. One use of the CONNID is in establishing request priority among various initiators. Another use is in associating actions or data transfers with initiator identity rather than the address presented with the transaction request.

[0123] The embodiment of FIG. 11 provides end-to-end connection identification with CONNID as well as point-to-point, or more local identification with Thread ID. A Thread ID is an identifier of local scope that simply identifies transfers between the interface module and its connected functional block. In contrast, the CONNID is an identifier of global scope that identifies transfers between two interface modules (and, if required, their connected functional blocks).

[0124] A Thread ID should be small enough to directly index tables within the connected interface module and functional block. In contrast, there are usually more CONNIDs in a system than any one interface module is prepared to simultaneously accept. Using a CONNID in place of a Thread ID requires expensive matching logic in the interface module to associate a returned CONNID with specific requests or buffer entries.

[0125] Using a networking analogy, the Thread ID is a level-2 (data link layer) concept, whereas the CONNID is more like a level-3 (transport/session layer) concept. Some functional blocks only operate at level-2, so it is undesirable to burden the functional block or its interface module with the expense of dealing with level-3 resources. Alternatively, some functional blocks need the features of level-3 connections, so in this case it is practical to pass the CONNID through to the functional block.

[0126] Referring to FIG. 4, a CONNID should be unique when transferred between interface modules 1004 and 1006 on shared communications bus 1012. The CONNID may be sent over a local interconnect, such as interconnect 1010. In many cases, however, it is much more efficient to use only Thread ID between a functional block and its interface module. For example initiator functional block 1002 may not require all the information provided by the CONNID. Also, in some systems, multiple identical initiator functional blocks 1002 may exist with the same CONNID so that a particular target functional block 1008 receiving a transfer will not know which connection it is actually part of unless logic in initiator interface module 1004 translates the “local” CONNID to a unique “global” CONNID. The design and implementation of such a translation functionality in an interface module is complicated and expensive. In such cases, the CONNID may be sent between interface modules over shared communications bus 1012 while the Thread ID is sent between a functional block and an interface module.

[0127] In the case of an initiator functional block, a one-to-one static correspondence may exist between Thread ID and CONNID. For example if the Thread ID is “1”, a single CONNID is mapped for a particular interface module, solving the problem of multiple, identical functional blocks.

[0128] In the case of a target functional block, there is a one-to-one dynamic correspondence between a Thread ID and a CONNID. If a target functional block supports two simultaneous threads, the target interface module acquires the CONNID of an open connection and associates it with a thread as needed. For example, a target interface module receives a CONNID of “7”, and then maps CONNID 7 to thread “0”. Thereafter, all transfers with CONNID 7 are associated with thread 0 until connection 7 is closed.

[0129] Referring to FIG. 12, an example of a use of Thread ID, consider a series of identical direct memory access (DMA) engines in a system. In FIG. 12, elements 1202 are identical DMA engines, each connected to an initiator interface module 1204. Initiator interface modules 1204 are connected to shared communications bus 1212. Target interface module 1206 is also connected to shared communications bus 1212 and transmits data from bus 1212 to DRAM controller 1208, which is a target functional block. Target interface module 1206 is connected to DRAM controller 1208 by interconnect 1214. DRAM controller 1208 controls access to DRAM 1213.

[0130] A DMA a engine is an example of an initiator functional block that also functions as a target functional block. When the DMA engine is programmed by software, it acts as a target. Thereafter, the DMA engine is an initiator. Because a DMA engine performs both read and write operations, two connections can be associated with a single DMA engine. If some buffering is available in the DMA engine, read and write operations may be decoupled so that both types of operations can be performed concurrently. A read may occur from a long latency storage device which requires the read data to be buffered on the DMA engine before a write operation writes the data. In one embodiment, each of DMA engines 1202 uses a Thread ID to identify the read stream and a different Thread ID to identify the write stream. The DMA engine does not require more information, such as what other functional block participates in a transaction. Therefore, a CONNID is not required to be sent from the DMA engine 1202 to a connected interface module 1204. Mapping of a Thread ID to a CONNID occurs in the interface module 1204.

[0131] In one embodiment, each initiator interface module 1204 maps a unique CONNID to each of two Thread Ids from a connected DMA engine 1202. Each of DMA engines 1202 use a single bit, for example, Thread ID of FIG. 11, to distinguish between its two threads. For each transfer over shared communications bus a unique CONNID is sent to target interface module 1206. The CONNID may include priority information, for example, assigning high priority to requests for graphics data. The high priority graphics data request is immediately serviced by DRAM controller 1208 while lower priority request may be required to wait.

[0132] Because intelligence is designed into the interface modules and the communications protocols, less intelligence is required of the functional block such as the DRAM controller 1208 and the DMA engines 1202. This has the advantage of making functional blocks more portable or reusable as systems evolve. For example, a DMA engine used for a high priority application may be switched with a DMA engine used for a lower priority application simply by changing their respective connected interface modules.

[0133] In one embodiment, target and initiator interface modules are programmed at the transistor level so that their precise function, including their CONNID assignment, is fixed at power-up. In another embodiment, the design of interface modules is in RAM so that the interface module is a reprogrammable resource. In this case, the interface module is reprogrammed, including reassignment of CONNIDs, by software.

[0134] The present invention has been described in terms of specific embodiments. For example, embodiments of the present invention have been shown as systems of particular configurations, including protocols having particular signals. One of ordinary skill in the art will recognize that different system configurations and different signals may be used without departing from the spirit and scope of the invention as set forth in the claims. 

What is claimed is:
 1. A method for communicating data between functional blocks in a computing device, comprising: establishing at least one thread identifier, each thread identifier associating a data transfer with a transaction stream that the data transfer between an initiator functional block and a target functional block are part of; if the target functional block is unable to accept a data transfer from the initiator functional block, the target functional block issuing a busy signal identified by the thread identifier; and the initiator functional block withholding issuance of data transfers associated with the thread identifier in response to the issued busy signal, wherein data transfers not associated with the thread identifier identified by the issued busy signal may be issued.
 2. The method as set forth in claim 1, wherein the busy signal comprises a signal that is maintained active when the target functional block is unable to accept data transfers.
 3. The method as set forth in claim 1, wherein the busy signal comprises a credit signal used to communicate a number of credits that indicate how many data transfers the target functional block can accept.
 4. The method as set forth in claim 3, further comprising decrementing the number of credits for each active data transfer and incrementing the number of credits upon freeing up of resources for further data transfers.
 5. The method as set forth in claim 3, wherein the credit signal is generated by maintaining the signal in an active state for a number of clock cycles corresponding to the number of credits.
 6. The method as set forth in claim 3, wherein the credit signal comprises a coded signal indicative of the number of credits.
 7. The method as set forth in claim 1, further comprising determining service guarantees for at least one transaction stream between initiator functional blocks and the target functional blocks.
 8. The method as set forth in claim 1, further comprising the initiator functional block stopping to send data transfers so that the target functional block receives no more than a determined number of data transfers after issuance of the busy signal.
 9. The method as set forth in claim 1, wherein the target functional block issues a busy signal no more than a determined number of clock cycles after the target functional block determines that it has insufficient buffer space to receive data transfers from an initiator functional block.
 10. The method as set forth in claim 8, further comprising the target device buffering the data transfers received after issuance of the busy signal until resources become available to service the buffered data transfers.
 11. The method as set forth in claim 7, wherein determining service guarantees comprises: mapping the transaction stream to data channels of components between an initiator device and target device; converting performance guarantees of selected data channels of the mapped transaction stream such that the guarantees of the data channels are aligned to be uniform in units; and aggregating the guarantees of the data channels for the transaction stream.
 12. The method as set forth in claim 11, wherein aggregating comprises a function selected from the group consisting of summing the guarantees of the data channels of the transaction stream, selecting the maximum guarantees of the data channels of the transaction stream, and selecting the minimum guarantees of the data channels of the transaction stream.
 13. The method as set forth in claim 11, wherein the guarantees are selected from the group consisting of quality of service guarantees, performance guarantees, bandwidth guarantees, latency guarantees, maximum outstanding request guarantees and maximum variance in service latency guarantees.
 14. A method for communicating data between functional blocks in a computing device, comprising: establishing at least one thread identifier, each thread identifier associating a data transfer with a transaction stream that the data transfer between an initiator functional block and a target functional block are part of; if the target functional block is unable to accept a data transfer from the initiator functional block, the target functional block issuing a busy signal identified by the thread identifier; and storing in a buffer data transfers received after issuance of the busy signal until resources become available to service the buffered data transfers, the amount of buffer sufficient to buffer any transfers that arrive after the busy signal is asserted, wherein an interface between the initiator functional block and target functional block does not block data transfers of other threads.
 15. The method as set forth in claim 14, wherein the target functional block issues a busy signal a determined number of clock cycles after the target functional block determines that it is unable to accept a data transfer from an initiator functional block.
 16. The method as set forth in claim 14, further comprising the target functional block receiving no more than a determined number of data transfers after issuance of the busy signal.
 17. The method as set forth in claim 14, further comprising determining service guarantees for at least one transaction stream between initiator functional blocks and the target functional blocks.
 18. The method as set forth in claim 17, wherein determining service guarantees comprises: mapping the transaction stream to data channels of components between an initiator device and target device; converting performance guarantees of selected data channels of the mapped transaction stream such that the guarantees of the data channels are aligned to be uniform in units; and aggregating the guarantees of the data channels for the transaction stream.
 19. The method as set forth in claim 18, wherein aggregating comprises a function selected from the group consisting of summing the guarantees of the data channels of the transaction stream, selecting the maximum guarantees of the data channels of the transaction stream, and selecting the minimum guarantees of the data channels of the transaction stream.
 20. A communication apparatus comprising: at least two functional blocks, wherein an initiator functional block communicates with a target functional block by establishing a connection; a bus coupled to each of the functional blocks and configured to carry a plurality of signals, wherein the plurality of signals comprises a thread identifier configured to associate a data transfer with a transaction stream between the initiator functional block and target functional block, and a busy signal identified by the thread identifier, the busy signal issued by the target functional block to indicate resources are unavailable to perform a data transfer, wherein the initiator functional block associated withholds issuance of data transfers associated with the thread identifier in response to the issuance of the busy signal.
 21. The apparatus as set forth in claim 20, wherein the busy signal comprises a signal that is maintained active when the target functional block is unable to accept data transfers.
 22. The apparatus as set forth in claim 20, wherein the busy signal comprises a credit signal comprising a number of credits that indicate how many data transfers the target functional block can accept.
 23. The apparatus as set forth in claim 22, wherein the number of credits is decremented for each active data transfer and incremented upon freeing up of resources for further data transfers.
 24. The apparatus as set forth in claim 22, wherein the credit signal is generated by maintaining the signal in an active state for a number of clock cycles corresponding to the number of credits.
 25. The apparatus as set forth in claim 22, wherein the credit signal comprises a coded signal indicative of the number of credits.
 26. The apparatus as set forth in claim 20, wherein the at least one transaction stream is non-blocking enabling determination of service guarantees for transaction streams between initiator functional blocks and target functional blocks.
 27. The apparatus as set forth in claim 20, wherein the target functional block further comprises a buffer to receive data transfers issued by the initiator functional block after issuance of the busy signal by the target functional block and before receipt of the busy signal by the initiator functional block.
 28. The apparatus as set forth in claim 27, wherein service guarantees are determined by mapping the transaction stream to data channels of components between an initiator device and target device, converting performance guarantees of selected data channels of the mapped transaction stream such that the guarantees of the data channels are aligned to be uniform in units, and aggregating the guarantees of the data channels for the transaction stream.
 29. The apparatus as set forth in claim 28, wherein aggregating comprises a function selected from the group consisting of summing the guarantees of the data channels of the transaction stream, selecting the maximum guarantees of the data channels of the transaction stream, and selecting the minimum guarantees of the data channels of the transaction stream.
 30. The apparatus as set forth in claim 26, wherein the guarantees are selected from the group consisting of quality of service guarantees, performance guarantees, bandwidth guarantees, latency guarantees, maximum outstanding request guarantees and maximum variance in service latency guarantees.
 31. A communication apparatus comprising: at least two functional blocks, wherein an initiator functional block communicates with a target functional block by establishing a connection; a bus coupled to each of the functional blocks and configured to carry a plurality of signals, wherein the plurality of signals comprises at least one thread identifier configured to associate a data transfer with a transaction stream that the data transfer between an initiator functional block and a target functional block are part of; wherein if the target functional block is unable to accept a data transfer from the initiator functional block, the target functional block issuing a busy signal identified by the thread identifier and buffering data transfers received after issuance of the busy signal until resources become available to service the buffered data transfers; and a buffer coupled to the target functional block, the size of the buffer sufficient to buffer any data transfers that arrive on the transaction stream after the busy signal is asserted, wherein an interface between the initiator functional block and target functional block does not block the interface and data transfers of at east one of other transactions streams can be performed.
 32. The apparatus as set forth in claim 31, wherein the target functional block issues a busy signal a determined number of clock cycles after the target functional block determines that it is unable to accept a data transfer from an initiator functional block.
 33. The apparatus as set forth in claim 31, further comprising the target functional block receiving no more than a determined number of data transfers after issuance of the busy signal.
 34. The apparatus as set forth in claim 31, further comprising the target functional block determining service guarantees for at least one transaction stream between initiator functional blocks and the target functional blocks.
 35. The apparatus as set forth in claim 34, wherein determining service guarantees comprises: mapping the transaction stream to data channels of components between an initiator device and target device; selectively converting determine guarantees of data channels of components of the mapped transaction stream such that the guarantees of the data channels are aligned to be uniform in units; and aggregating the guarantees of the data channels for the transaction stream.
 36. The apparatus as set forth in claim 35, wherein aggregating comprises a function selected from the group consisting of summing the guarantees of the data channels of the transaction stream, selecting the maximum guarantees of the data channels of the transaction stream, and selecting the minimum guarantees of the data channels of the transaction stream. 